# Copyright 2013, Michael H. Goldwasser
#
# Developed for use with the book:
#
#    Data Structures and Algorithms in Python
#    Michael T. Goodrich, Roberto Tamassia, and Michael H. Goldwasser
#    John Wiley & Sons, 2013
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.

from ..exceptions import Empty


class CircularQueue:
	"""Queue implementation using circularly linked list for storage."""

	# ---------------------------------------------------------------------------------
	# nested _Node class
	class _Node(object):
		"""Lightweight, nonpublic class for storing a singly linked node."""
		__slots__ = '_element', '_next'  # streamline memory usage

		def __init__(self, element, next):
			self._element = element
			self._next = next

	# end of _Node class
	# ---------------------------------------------------------------------------------

	def __init__(self):
		"""Create an empty queue."""
		self._tail = None  # will represent tail of queue
		self._size = 0  # number of queue elements

	def __len__(self):
		"""Return the number of elements in the queue."""
		return self._size

	def is_empty(self):
		"""Return True if the queue is empty."""
		return self._size == 0

	def first(self):
		"""Return (but do not remove) the element at the front of the queue.

		Raise Empty exception if the queue is empty.
		"""
		if self.is_empty():
			raise Empty('Queue is empty')
		head = self._tail._next
		return head._element

	def dequeue(self):
		"""Remove and return the first element of the queue (i.e., FIFO).

		Raise Empty exception if the queue is empty.
		"""
		if self.is_empty():
			raise Empty('Queue is empty')
		oldhead = self._tail._next
		if self._size == 1:  # removing only element
			self._tail = None  # queue becomes empty
		else:
			self._tail._next = oldhead._next  # bypass the old head
		self._size -= 1
		return oldhead._element

	def enqueue(self, e):
		"""Add an element to the back of queue."""
		newest = self._Node(e, None)  # node will be new tail node
		if self.is_empty():
			newest._next = newest  # initialize circularly
		else:
			newest._next = self._tail._next  # new node points to head
			self._tail._next = newest  # old tail points to new node
		self._tail = newest  # new node becomes the tail
		self._size += 1

	def rotate(self):
		"""Rotate front element to the back of the queue."""
		if self._size > 0:
			self._tail = self._tail._next  # old head becomes new tail
